Floating liquefied natural gas pretreatment system

ABSTRACT

A pretreatment system and method for a floating liquid natural gas (“FLNG”) facility are presented. The inlet natural gas stream flows through a membrane system to remove carbon dioxide and a heat exchanger, producing first and second cooled CO 2 -depleted non-permeate streams. The first cooled CO 2 -depleted non-permeate stream is routed to additional pretreatment equipment, while the second cooled CO 2 -depleted non-permeate stream is routed directly to a LNG train. Alternatively, the inlet natural gas stream may flow through a membrane system to produce a single cooled CO 2 -depleted non-permeate stream that is routed to the LNG train after sweetening and dehydration. Because the pretreatment system delivers the incoming gas stream to the LNG train at a lower temperature than conventional systems, less energy is needed to convert the gas stream to LNG. In addition, the pretreatment system has a smaller footprint than conventional pretreatment systems.

CROSS-REFERENCE TO PENDING APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/567,406, filed on Dec. 6, 2011.

BACKGROUND OF THE INVENTION

Offshore production facilities use pipelines to transport natural gasextracted from deep sea gas fields or gas-laden oil-rich fields toprocessing facilities located on the nearest shore.

These onshore facilities chill the natural gas, which turns the gas intoa liquid and shrinks its volume by about 600 times. This chilling isaccomplished by a liquefied natural gas (“LNG”) refrigeration train (or,simply, an LNG train). The LNG produced by the train is then loaded ontoships for transportation to customers. Alternatively, in a verydifferent and new processing approach, the natural gas is processed intoLNG at sea on board a ship. Such ships are known as floating liquefiednatural gas (“FLNG”) facilities.

Regardless of whether the gas is being liquefied at an onshore facilityor at sea, some pretreatment of the incoming natural gas stream isrequired to remove contaminants, such as water, carbon dioxide, hydrogensulfide, and mercury, so that the gas stream being processed by the LNGtrain is about 99 percent pure gas. This pretreatment is usuallyaccomplished by way of an amine unit, which removes hydrogen sulfide andcarbon dioxide, and a dehydration unit, which removes water. Thepretreatment system then delivers the substantially pure natural gasstream to the LNG train at a temperature of about 37° C. to 49° C. (100°F. to 120° F.).

Energy consumption is a key cost driver in all LNG trains. Inparticular, the natural gas stream must be cooled or refrigerated toabout −160° C. (−256° F.) to make LNG. Because the refrigeration processconsumes more energy than any other process in the LNG train, energyoptimization and conservation are key design considerations. The volumesof gas being treated are typically very large, so even small decreasesin the temperature of the incoming gas stream to the LNG train maytranslate into significant energy and cost savings.

A FLNG train has additional challenges in this regard because of itslocation offshore and the limited amount of space available aboard ship.For example, the footprint of an FLNG train may be about one-fourth thesize of an onshore LNG processing facility with equivalent capacity. Aneed exists for a pretreatment system that has a smaller footprint thanconventional pretreatment systems and that can deliver the incoming gasstream to the LNG train at a lower temperature, thereby reducing theamount of energy required for the refrigeration process.

SUMMARY OF THE INVENTION

An embodiment of a pretreatment system for a FLNG facility is presented.The pretreatment system includes a membrane system that removes carbondioxide from an inlet natural gas stream, producing a cooled CO₂-richpermeate stream and a cooled CO₂-depleted non-permeate stream. Thesystem also includes a heat exchanger that cross-exchanges heat from thecooled non-permeate and permeate streams with the substantiallywater-free natural gas outlet stream to produce a first cooledCO₂-depleted non-permeate stream and a second cooled CO₂-depletednon-permeate stream. The first cooled CO₂-depleted non-permeate streamis routed to additional pretreatment equipment, while the second cooledCO₂-depleted non-permeate stream is routed directly to a LNG train. Thepretreatment system may also include mercury removal, gas sweetening,and gas dehydration.

Another embodiment of a pretreatment system for a FLNG facility includesa mercury removal system that removes mercury from an inlet natural gasstream to produce a substantially mercury-free natural gas stream, amembrane system that removes carbon dioxide from the substantiallymercury-free natural gas stream to produce a cooled CO₂-depletednon-permeate stream, a gas sweetening system that processes the cooledCO₂-depleted non-permeate stream to form a sweetened natural gas stream,and a gas dehydration system that removes water from the sweetenednatural gas stream to form a substantially water-free natural gas outletstream. The substantially water-free natural gas outlet stream is fed toa LNG train. A pretreatment method for cooling and purifying a naturalgas stream so that it can be processed into LNG is also presented. Thesteps of the method include passing an inlet natural gas stream througha membrane system to produce a cooled CO₂-rich permeate stream and acooled CO₂-depleted non-permeate stream, cross-exchanging heat from thenon-permeate and permeate streams and a substantially water-free naturalgas outlet stream in a heat exchanger to produce a first cooledCO₂-depleted non-permeate stream and a second cooled CO2-depletednon-permeate stream, routing the first cooled CO₂-depleted non-permeatestream to additional pretreatment equipment, and directing the secondcooled CO₂-depleted non-permeate stream directly to a LNG train.Additional processing steps may include mercury removal, gas sweetening,and gas dehydration.

Objects of the invention are to (1) conserve energy by reducing theamount needed by the LNG train to refrigerate natural gas; (2) improveenergy efficiency of the LNG train; (3) reduce capital equipment andoperating costs; (4) reduce the design sizes of the equipment associatedwith certain steps in the pretreatment process; and (5) minimize theoverall amount of space necessary to convert the inlet natural gasstream to LNG.

A better understanding of the invention will be obtained from thefollowing detailed description of the preferred embodiments taken inconjunction with the drawings and the attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described in furtherdetail. Other features, aspects, and advantages of the present inventionwill become better understood with regard to the following detaileddescription, appended claims, and accompanying drawings (which are notto scale) where:

FIG. 1 is a general layout of a FLNG facility showing a pretreatmentsystem located upstream of the LNG train, according to an embodiment ofthe present invention.

FIG. 2 is a simplified process flow diagram of a preferred embodiment ofa pretreatment system, according to an embodiment of the presentinvention. The pretreatment system purifies and cools the inlet naturalgas stream before it enters the LNG train, and is well-suited for use inthe FLNG facility of FIG. 1.

FIG. 3 is a simplified process flow diagram of another preferredembodiment of a pretreatment system, according to an embodiment of thepresent invention. The pretreatment system of FIG. 3 is also well-suitedfor use in the FLNG facility of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is to be understood that the invention that is now to be described isnot limited in its application to the details of the construction andarrangement of the parts illustrated in the accompanying drawings. Theinvention is capable of other embodiments and of being practiced orcarried out in a variety of ways. The phraseology and terminologyemployed herein are for purposes of description and not limitation.

Elements shown by the drawings are identified by the following numbers:

-   10 From FLNG production/topside-   19 Inlet natural gas stream-   20 Mercury removal system-   21 Substantially mercury-free natural gas stream-   30 Membrane system-   31 Cooled CO₂-rich permeate stream-   33 Cooled CO₂-depleted non-permeate stream-   40 Heat exchanger-   41 First cooled CO₂-depleted non-permeate stream-   43 Second cooled CO₂-depleted non-permeate stream-   45 Heated CO₂-rich permeate stream-   50 Gas sweetening system-   51 Sweetened natural gas stream-   60 Gas dehydration system-   61 Substantially water-free natural gas outlet stream-   70 To LNG train-   71 Single cooled CO₂-depleted non-permeate stream-   80 LNG gathering system

This invention describes a pretreatment system that delivers theincoming gas stream to the LNG train at a lower temperature thanconventional systems. As a result, less energy is needed to convert thegas stream to LNG. In addition, the pretreatment system has a smallerfootprint than conventional pretreatment systems.

As shown in FIG. 1, a FLNG system aboard a sea-going vessel includesareas for power generation and utilities, pretreatment of the incomingnatural gas stream, liquefaction of the natural gas into LNG, and LNGstorage. The system also includes means for flaring excess natural gasand means, such as a cryogenic export system, for transferring LNG fromthe vessel to the next point of delivery. The pretreatment system may belocated anywhere on the vessel but is generally placed near the LNGtrain. FIG. 2 and FIG. 3 show potential embodiments of the pretreatmentsystem described herein.

As shown in FIG. 2, a pretreatment system to cool and purify the inletnatural gas stream may include multiple components. One component may bea mercury removal system 20, which may be a PURASPEC_(JM)™ mercury/H₂Sremoval bed (Johnson Matthey Catalysts, Houston, Tex.) or itsequivalent. The mercury removal system 20 removes the mercury from theinlet natural gas stream 19 to form a substantially mercury-free naturalgas stream 21. The substantially mercury-free natural gas stream 21 maythen be passed through a membrane system 30. Alternatively, themercury/H₂S removal bed may be installed downstream of the gasdehydration system 60.

The membrane system 30 may be primarily responsible for removing carbondioxide from the substantially mercury-free natural gas stream 21. As anexample, the membrane system 30 is preferably one or more CYNARA® CO₂removal membrane systems (Cameron Process Systems, Houston, Tex.) or itsequivalent. As the substantially mercury-free natural gas stream 21passes through the membrane system 30, it is naturally cooled to producea cooled CO₂-rich permeate stream 31 and a cooled CO₂-depletednon-permeate stream 33. Both non-permeate and permeate streams 31, 33may then be fed to a heat exchanger 40.

In the heat exchanger 40, heat from the cooled CO₂-rich permeate stream31 and the cooled CO₂-depleted non-permeate stream 33 is cross-exchangedwith a substantially water-free natural gas outlet stream 61 from a gasdehydration system 60 to produce a first cooled CO₂-depletednon-permeate stream 41, a second cooled CO₂-depleted non-permeate stream43, and a heated CO₂-rich permeate stream 45. The heated CO₂-richpermeate stream 45 leaves the pretreatment system and is used as fuel orcan be reinjected.

After leaving the heat exchanger 40, the first cooled CO₂-depletednon-permeate stream 41 may pass through a gas sweetening system 50 thatproduces a sweetened natural gas stream 51. As an example, the gassweetening system 50 may be a hybrid system that includes a membranesystem for removing carbon dioxide and hydrogen sulfide followed by anamine unit for removing carbon dioxide and hydrogen sulfide to the levelrequired by the downstream refrigeration unit. The sweetened natural gasstream 51 may then pass through a gas dehydration system 60. The gasdehydration system 60 removes water from the sweetened natural gasstream 51 to produce a substantially water-free natural gas outletstream 61 which is fed to the heat exchanger 40. The gas dehydrationsystem 60 may include, but is not limited to, molecular sieves andhydrocarbon dew pointing.

The second cooled CO₂-depleted non-permeate stream 43 may leave the heatexchanger 40 and be routed directly to a LNG train 70. Because thesecond cooled CO₂-depleted non-permeate stream 43 has been cooled by themembrane system 30 and further cooled by the heat exchanger 40, thetemperature at which it is delivered to the LNG train 70 issignificantly reduced when compared to conventional pretreatmentsystems. For example, the temperature of the second cooled CO₂-depletednon-permeate stream 43 may be about 15° C. to 21° C. (60° F. to 70° F.),while the temperature at which a conventional pretreatment systemdelivers natural gas to a LNG train is about 37° C. to 48° C. (100° F.to 120° F.). Delivering the second cooled CO₂-depleted non-permeatestream 43 to the LNG train 70 at a cooler temperature reduces theoverall LNG refrigeration load and the amount of energy required.

As another example, the temperature of the outlet gas from aconventional pretreatment system may range from about 41° C. to 43° C.(105° F. to 110° F.). A dual-mixed refrigerant system with a designcapacity of two million tons per annum (“mtpa”) LNG requires 62megawatts (“MW”) of energy to convert outlet gas at that temperature toLNG. In contrast, as shown in the table below, the outlet gastemperature for the pretreatment system of the present invention rangesfrom about 26° C. to 35° C. (78° F. to 95° F.) as carbon dioxide inletand outlet concentrations vary:

Feed Gas Product Gas Predicted Outlet Predicted Outlet CO₂ CO₂ GasTemperature Gas Temperature (mol %) (mol %) (° C.) (° F.) 20 7 26 78 157 29 84 10 7 32 89 7 3 33 91 5 3 35 95Because the temperature of the outlet gas is lower, the LNG trainrequires less energy to convert the outlet gas to LNG. This energysavings when compared to the conventional pretreatment system is aboutfive to fifteen percent.

Like the second cooled CO₂-depleted non-permeate stream 43, the firstcooled CO₂-depleted non-permeate stream 41 may also pass through themembrane system 30 and the heat exchanger 40, reducing its temperaturebefore it enters the gas dehydration system 60. In addition, the secondcooled CO₂-depleted non-permeate stream 43 entirely bypasses the gasdehydration system 60. As a result, the gas dehydration system 60 has totreat less gas, and the gas that it does treat is at a lowertemperature. The size of the gas dehydration system 60 may therefore bereduced, and the pretreatment system requires less floor space thanconventional pretreatment systems.

As an alternative, the first and second cooled CO₂-depleted non-permeatestreams 41, 43 may be used to cool the inlet natural gas stream 19, thesubstantially mercury-free natural gas stream 21, or both. If thenatural gas is rich in heavy hydrocarbons, this cooling results in heavyhydrocarbon and water condensation in the inlet gas stream. This reducesthe amount of water and heavy hydrocarbons that enter the pretreatmentsystem, thereby reducing the overall processing load on the pretreatmentsection of the FLNG system.

An alternate embodiment of a FLNG pretreatment system to cool and purifyan inlet stream of natural gas is shown in FIG. 3. The inlet natural gasstream 19 from the LNG gathering system 80 may first be sent to amercury removal system 20, which removes the mercury from the inletnatural gas stream 19 to form a substantially mercury-free natural gasstream 21. The substantially mercury-free natural gas stream 21 may thenbe passed to a membrane system 30, which is primarily responsible forremoving carbon dioxide from the substantially mercury-free natural gasstream 21. The membrane system 30 also naturally cools the substantiallymercury-free natural gas stream 21 to produce a single cooledCO₂-depleted non-permeate stream 71. The single cooled CO₂-depletednon-permeate stream 71 may then pass through a gas sweetening system 50that produces a sweetened natural gas stream 51. The sweetened naturalgas stream 51 may then pass through a gas dehydration system 60, whichremoves water from the sweetened natural gas stream 51 to produce asubstantially water-free natural gas outlet stream 61 which is fed tothe LNG train 70. Passing through the membrane system 30 reduces thetemperature of the single cooled CO₂-depleted non-permeate stream 71. Asa result, downstream treatment equipment may be reduced in size and lessenergy is required to convert the substantially water-free natural gasoutlet stream 61 to LNG.

A FLNG pretreatment system and method have been disclosed. While theinvention has been described with a certain degree of particularity, itis manifest that many changes may be made in the details ofconstruction, the types and arrangement of components, and the numberand order of pretreatment steps without departing from the spirit andscope of this disclosure. It is understood that the invention is notlimited to the embodiments set forth herein for purposes ofexemplification, but is to be limited only by the scope of the attachedclaims, including the full range of equivalency to which each elementthereof is entitled.

What is claimed is:
 1. A pretreatment system for a floating liquidnatural gas (“FLNG”) facility comprising a membrane system that producesa cooled CO₂-rich permeate stream and a cooled CO₂-depleted non-permeatestream by removing carbon dioxide from an inlet natural gas stream; anda heat exchanger that cross-exchanges heat from the cooled CO₂-richpermeate stream and the cooled CO₂-depleted non-permeate stream with asubstantially water-free natural gas outlet stream to produce a firstcooled CO₂-depleted non-permeate stream and a second cooled CO₂-depletednon-permeate stream wherein the first cooled CO₂-depleted non-permeatestream is routed to additional pretreatment equipment and the secondcooled CO₂-depleted non-permeate stream is routed to a LNG train.
 2. Apretreatment system according to claim 1 further comprising a mercuryremoval system that processes the inlet natural gas stream to form asubstantially mercury-free natural gas stream.
 3. A pretreatment systemaccording to claim 1 wherein the additional pretreatment equipment ischosen from the group consisting of a gas sweetening system and a gasdehydration system.
 4. A pretreatment system according to claim 1wherein the membrane system is a CO₂ removal membrane system.
 5. Apretreatment system according to claim 3 wherein the gas sweeteningsystem is comprised of a membrane system for removing carbon dioxide andan amine unit for removing hydrogen sulfide.
 6. A pretreatment systemaccording to claim 1 wherein the second cooled CO₂-depleted non-permeatestream has a temperature ranging from about 15° C. to 40° C. (60° F. to104° F.) when it is delivered to the LNG train.
 7. A pretreatment systemaccording to claim 6 wherein the second cooled CO₂-depleted non-permeatestream has a temperature ranging from about 15° C. to 21° C. (60° F. to70° F.) when it is delivered to the LNG train.
 8. A pretreatment systemaccording to claim 1 wherein the first cooled CO₂-depleted non-permeatestream and the second cooled CO₂-depleted non-permeate stream are usedto cool the inlet natural gas stream.
 9. A pretreatment system accordingto claim 2 wherein the first cooled CO₂-depleted non-permeate stream andthe second cooled CO₂-depleted non-permeate stream are used to cool thesubstantially mercury-free natural gas stream.
 10. A pretreatment systemfor a floating liquid natural gas (“FLNG”) facility comprising a mercuryremoval system that removes mercury from an inlet natural gas stream toproduce a substantially mercury-free natural gas stream; a membranesystem that removes carbon dioxide from the substantially mercury-freenatural gas stream to produce a cooled CO₂-depleted non-permeate stream;a gas sweetening system that receives the cooled CO₂-depletednon-permeate stream and processes it to form a sweetened natural gasstream; and a gas dehydration system that removes water from thesweetened natural gas stream to form a substantially water-free naturalgas outlet stream which is fed to a LNG train.
 11. A pretreatment systemaccording to claim 10 wherein the mercury removal system is amercury/H₂S removal bed.
 12. A pretreatment system according to claim 11wherein the mercury removal system is a PURASPEC_(JM)™ mercury/H₂Sremoval bed.
 13. A pretreatment system according to claim 10 wherein themembrane system is a CO₂ removal membrane system.
 14. A pretreatmentmethod for cooling and purifying a natural gas stream for processinginto LNG, the method comprising the steps of: passing an inlet naturalgas stream through a membrane system to produce a cooled CO₂-richpermeate stream and a cooled CO₂-depleted non-permeate stream;cross-exchanging heat from the cooled CO₂-rich permeate stream and thecooled CO₂-depleted non-permeate stream with a substantially water-freenatural gas outlet stream in a heat exchanger to produce a first cooledCO₂-depleted non-permeate stream and a second cooled CO2-depletednon-permeate stream; routing the first cooled CO₂-depleted non-permeatestream to additional pretreatment equipment; and directing the secondcooled CO₂-depleted non-permeate stream directly to a LNG train.
 15. Apretreatment method according to claim 14 further comprising the step ofprocessing the inlet natural gas stream in a mercury removal system toform a substantially mercury-free natural gas stream.
 16. A pretreatmentmethod according to claim 15 wherein the mercury removal system is amercury/H₂S removal bed.
 17. A pretreatment method according to claim 16wherein the mercury removal system is a PURASPEC_(JM)™ mercury/H₂Sremoval bed.
 18. A pretreatment method according to claim 14 wherein theadditional pretreatment equipment is chosen from the group consisting ofa gas sweetening system and a gas dehydration system.
 19. A pretreatmentmethod according to claim 14 wherein the membrane system is a CO₂removal membrane system.
 20. A pretreatment method according to claim 15wherein the mercury removal system is located after the additionalpretreatment equipment.